Instructables
Picture of $4 Solar Battery Charger

When I got into electrical circuits and solar power, the first thing I wanted to do was build a little solar powered battery charger. Only I had a heck of a time trying to find a simple and straight forward guide to doing this.

So in this guide I'll give you a bit of info on solar power and battery charging, as well as show you how to make a solar battery charger for all of $4.

If you'd like some solar panels or solar kits I have quite a few on my gadget site, browndoggadgets.com or you can also buy them off ebay or various other websites.

 
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Step 1: What You Need

To build a solar battery charger you need several things, as well as have several tools on hand.

Parts
A clear, water-proof container. (Dollar Store tupperware with built in O-Ring)
AA Battery Holder (Radio Shack, also fits AAAs if you're careful)
One or Two Solar Panels rated 4 Volts or above
Blocking Diode
Tools you need
Soldering Iron
Solder
Tape
Safety Goggles
Some wire

Time: 20-30 minutes

Difficulty: Easy

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rishichavda1 month ago

Very nice project, could have made it very smaller and thus more portable/convenient. Great make overall, hopefully make my own in the next few days/weeks.

vuralzone2 months ago

Nice job !!

But I have question, do we really need the blocking diode?

As far as I know, solar panels have almost the same physical structure with diodes and they don't allow back current.

Could you correct me please

Thanks

kolotour7 months ago

Hey boys and girls, here's a source for some cheap solar battery chargers. Used solar powered yard lights. These ARE solar powered battery chargers already along with a circuit to turn the light on when it's dark. Folks toss these things when they stop working. Usually it's the battery that dies or corrosion that causes the failure. Many end up at your local 2nd hand store. Maybe 50 cents or less for each one. Insert a new battery and/or clean up the contacts and pull out the LED bulb you're done. They're inexpensive when new too. $2 - 4 bucks each.

conscious8 months ago
Hey, kids. Here are some electricity basics. mAh (milliAmpere-hour), a unit of charge and mA (milliAmpere), a unit of current (charge per unit time) are not the same thing. A 3000 mAh battery would run, in principle, for 30 hours if a constant current of 100 mA is drawn from it, for example.
Jobo178 months ago
I can't get over this charger, it's just "different" from the others, this is what got me wanting to make one, so whenever I pass by this when looking for a good design (which I'll probably just end up making my own), I just always click on it and look at it again and just see how simple it really is

P.S. My Algebra teacher in 8th grade had a last name of Zimmerman, so I always think of him when I see yours
Jobo178 months ago
mpeterson19, I can't tell if you mean on/off switch or have the electricity go one way? If your talking about direction then use a diode inbetween the panel and the circuitry (so it goes panel, wire, diode, wire, circuit stuff). If you're talking about on/off switch for charging then you would put an on/off switch between circuit and output or between panel circuit (in order)
mpeterson199 months ago
Could a charge controller be placed somewhere between the solar panels and the batteries? And can the batteries be charged while they are also powering a small device?
JCG59 months ago
Where did you get the solar panels from?
Jobo171 year ago
AndroidJack... it's the mA... the panel's mA shouldn't exceed 10% of the mA capacity of the battery...(theoretically)... the panel usual doesn't get what it says when it comes to mA so you have a little wiggle room.
corbinearl1 year ago
where's the best place to find the solar panels? I'm sorry if someone asked this and I didn't see it when I looked
Awesome build!

This really got me thinking, so I visited browndoggadgets.com for even more ideas. I have two 5V 100mA solar panels already that I want to use, but my question is what determines the rate at which the batteries charge? Is it the difference between the mA from the panels and the mA capacity of the batteries, is it something to do with the voltage, or is it a combination of both, and if so how exactly?
Great Instructable.

Just wondering how much the voltage of the solar cells matters? You went into depth about the amps, but only mentioned that your cells run at 4v.
Does the voltage need to be higher than the battery supply?

Also what happens if you do leave them to charge for months? I have some ideas for projects, but they would be sitting outside for a long time...
Yes, the charge voltage must exceed the total full-charge voltage of the batteries.

As long as you have your diode properly in place, keeping this setup out for a few weeks should be ok, but months may be pushing it, as the batteries are being charged and charged and charged without any usage save for leakage. In your case, this sounds unavoidable, so you may want to look into batteries that are built for this kind of duress, or even consider some type of device that switches off the solar panels when the batteries reach full capacity, and then on again when they become low.
JCG51 year ago
On my battery clip I have a toggle switch. When I have the solar panel charging the batteries, do I leave the switch on or off?
Depends on where the switch is located. If it is switching between the panels and the batteries, then leave it closed (on) so that the panels may charge the batteries. If it is after the batteries, then it really doesn't matter, unless there is a device plugged into the USB. In that case, close the switch to charge the device, and open it to prevent the device from charging. Are you using the switch INSTEAD of a diode? If that is the case, keep the switch closed only when you know the panels can harness sunlight.
Fashiondez1 year ago
Is it okay if I want to hook my two 4.5v 80mAs in a series for just two batteries, to make it more powerful?
GeoGyrl2 years ago
I am doing some research that requires about 300 AA batteries in a village with no electricity. I want to use the eneloop 2000 as it seems like they will last a long time and also recharge a lot of times. I decided to make solar chargers because $30 each for 30 chargers ON TOP of the battery costs is tough. I'll be right near the equator so sun isn't an issue. Been reading what I can, but what would you EXPERTS recommend to charge 4 AA batteries in about 8 hours? I'm guessing over 6V panel but what ma? I'm thinking of training women to make these little chargers for batteries and cellphones so trying to figure out how to solder without electricity. MAYBE can use butane if I can find a can in the city to recharge the cartridge. I leave in less than three weeks so any help would be appreciated.
For soldering u can use any kine of metal with sharp tip... Heated with any kind of heat source, fire from wood chips would work, nor biogas, kerosene stove, alcohol burner and many more....
Dbaby66 GeoGyrl2 years ago
You can find solder guns that use gas to burn. I have one. They are portable and use lighter fluid. You can find them on ebay pretty cheap.
You can get some that you actually put a lighter in. Thats a lot easier than trying to find lighter fluid
I will be finding on of those because that is way easier. Thanks.
GeoGyrl Dbaby662 years ago
It was difficult to find small containers of butane over there in Uganda and I couldn't take them on the plane so I bought battery powered soldering irons that used 4 AA batteries and they worked just fine. They aren't using them a lot, just a couple of connections. I was able to get solar panels and battery holders cheap and used eneloop rechargable batteries and it went great. Thanks for your ideas!
Dbaby66 GeoGyrl2 years ago
They can also use butane, mine just uses lighter fluid
Fashiondez2 years ago
will two 4.5v solar cells be able to efficiently charger four AAs? Or is it best to stay with just 2 AAs for that?
JoshuaZimmerman (author)  Fashiondez2 years ago
4 AAs equals 5V, so two of the 4.5V cells should work just fine.
bigjohn1a2 years ago
Great project! I

'm now having a problem figuring out how to rig one of these up to charge 4AA's that are connected to 8 LED's and have a dusk to dawn circuit. I have 2 panels that are 4.5 volts +/-, 3.2 volt-20ma Amber LED's, but no charging circuit. I think it's best in the long run to have a full charging circuit for this project. Do you think I am correct? I also don't have a properly configured dusk to dawn circuit. Any idea's? Any help would be great. Thanks.
Well, Sir: I think teachers should get a lot more respect, hence the Sir or Maam as appropriate. Anyone who masters E=I X R,and all its permutations can cut a wider swath through life. My jack of many trades abilities place me well below those of the proffesionals. Still, I am happy with where I am at. I really do think that little electronics equation is way more practical than most others. E = MCsquard is way less important for practical applications. Master those five little symbols and everything about them, there is no telling how far one can go. Thanks for your effort.
naturefreak2 years ago
Will a small led work as a blocking diode ?
kz1o3 years ago
It's worth mentioning that the "mah" unit you are mentioning is for the battery, and not for the charger. It's like the difference between "kilowatt hours", and "kilowatt". The first one might be how much energy a light bulb uses in a month, and the second one is just how much the light bulb uses. Some people compare this to a hose filling up a bucket with water. The size of the hose is how fast the water can go into it, and the size of the bucket is how much water it holds. Think of how many hose-seconds it takes to fill up a bucket.

Your charger circuit here will have the desired effect, and for the batteries you mention, the issues with milliamps should not be a concern. The kind of series (blocking) diode is not a big deal either, since the forward drop will not matter. You may use any garden variety silicon diode, and worst case is that the diode will fail (without damaging anything else) and you are out a penny. Just be sure to pick a diode that will allow the max current from the solar cell, in your case, 80 mA, which is 0.08 Amps.

Good luck, and always, safety first. It's a great habit to have.
DualPhase kz1o2 years ago
"Hose-seconds", lol. great example. Will ANY diode work, zener included? Are there any benefits of one over another?
JoshuaZimmerman (author)  kz1o3 years ago
Oh goodness. I can't believe I made that mistake. D'oh!

I really shouldn't be writing instructables at 1 a.m. anymore.

Thanks!
I disagree! Write them whenever you feel the motivation.... those creative juices flow at the most insane times of day or night, that is for sure.

The biggest tragedy would be for someone to have a great idea and not share it.
Vinsu kz1o3 years ago
Good instructable, JoshuaZimmerman! Nothing to do with this one but I guess that I'm one of those people that doesn't have guts to comment if something is annoying me. Don't know if it's a bad thing not to comment everything that comes into my mind because most of those disrespectful things are not constructive at all. I find it that myself should not say anything if it isn't "nice" or constructive. "How many usb-resistor-led-things do we need?"-comments are the ones that are the most hard to not to write when I feel the motivation... Flew! Glad got that out of my system. :)
ac-dc kz1o3 years ago
Actually the diode type used does matter. For example if you are charging 2 x NiMH cells in series you will reach a peak charge voltage of approximately 2.9V as 1.45V/cell, BUT we have to consider the sum nominal cell voltage over most of its recharge cycle, 2.4V as 1.2V/cell.

Now, if you have a silicone diode with about 0.6V forward drop, you have to produce 3V. If you have a schottky diode with 0.3V drop you only need to produce 2.7V.

3/2.7 = 1.11, an 11% efficiency increase. More importantly, during periods of less sunlight your cells may not even be able to get the voltage up to 3V at a meaningful charge current so that extra 0.3V margin could allow for charging for more hours per day too.
kz1o ac-dc3 years ago
You are right. My comment was made based on the statement made in the original post:

"One or Two Solar Panels rated 4 Volts or above"

If the 4-volt requirement is no longer true, then yes, it does matter.

techno guy3 years ago
Why hook them up in parallel? Wouldnt it be better in sieries?
JoshuaZimmerman (author)  techno guy3 years ago
Hooking the up in a Series would double the voltage. 4.5 + 4.5 = 9 volt, but keeping the current the same at 80 am.

Hooking up in Parallel will double the current, 80 ma + 80 ma = 180 ma, but keep the voltage at 4.5 volts.

It works the same for any power source. Try and imagine the solar panels as big barrels of water. In a series the two barrels of water are hooked up one on top of the other. They have twice as much water pressure source, but can only squeeze 80 ma of water out at a time through the one pipe.

In parallel you have the who barrels side by side, each pumping out 80 ma of water, doubling the amount of water coming out, but only one barrels worth of pressure.
This is the best explanation I have ever heard,.. truly genius,
just a quick question though, I have 5 solar cells, 2V 50mA each, I want 6 volts and 100mA I worked out how to do 2 in parallel and two in series, but I'm not sure where to stick the fifth one to increase the voltage.
you would put it in parallel with the series of the other 4 solar cells
JoshuaZimmerman (author)  NiKiToS3 years ago
To be honest, I'm not sure either. I'm pretty sure the most you're going to be able to do is 4V @ 100 ma. That extra one will limit the other two sets.

A solar panel is only as good as the least powerful cell. Which is why a single leaf falling on a solar panel can really mess up your output.
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